18 A COURSE ON ZOOLOGY. 



bonic acid gas, which, as we have seen, is rejected in ex- 

 piration. It is the tissues then that respire, and for them 

 that the blood passes to the lungs where it becomes 

 charged with oxygen. The tissues do not all respire 

 with the same intensity ; experiments have shown that 

 the muscles absorb the largest quantity of oxygen, while 

 the bones seem to require the least. 



In any case, as has before been said, respiration is 

 really a combustion, taking place in the midst of the 

 tissues, by the aid of oxygen carried by the blood. It 

 is well known that combustion produces heat, and it 

 can be understood why respiration is the principal source 

 of animal heat. 



CHAPTER III. 

 The Essential Phenomena of Circulation. 



Circulation is the name given to the course which the 

 blood follows through the system. The blood is a liquid 

 somewhat heavier than water, having an insipid taste 

 and an unpleasant odor ; its color varies between dark 

 red and scarlet. It is formed of two distinct parts, the 

 plasma, which is a colorless liquid, and round or flattened 

 globules, most of which are red, and these are called 

 corpuscles. The plasma forms eight hundred and sev- 

 enty-five thousandths of the blood, the corpuscles form 

 the other one hundred and twenty-five thousandths. 



The corpuscles are composed almost entirely of a sub- 

 stance closely resembling white of egg, and named, like 

 the latter, albumen ; their color is due to the presence 

 of a few thousandths of a substance containing iron, 



